This invention relates to an electrode, a method of fabricating the electrode, and a battery using the electrode. More particularly, it relates to an electrode whose resistivity changes with a rise in temperature, a method of fabricating the electrode, and a battery using the electrode.
In recent years, with the development of electronic equipment, batteries used therein as a power source have increasingly gained in capacity and output density. A lithium ion secondary battery is attracting attention as a battery fulfilling these requirements. A lithium ion secondary battery has an advantage of high energy density but requires sufficient measures for safety because of use of a nonaqueous electrolytic solution.
Conventionally proposed safety measures include a safety valve which relieves an increased inner pressure and a PTC element which increases resistivity on heat generation due to an external short-circuit to shut off the electric current. For example, incorporation of a safety valve and a PTC element into the cap of a positive electrode of a cylindrical battery is known as disclosed in JP-A-4-328278. However, on the safety valve""s working,moisture in the air enters the inside of the battery, which can induce an exothermic reaction in case lithium exists in the negative electrode.
On the other hand, a PTC element, which cuts off the external circuit involving a short-circuit, exerts no bad influence on operating. The PTC element can be designed to operate when the battery temperature rises to, for example, 90xc2x0 C. or higher due to an external short-circuit so as to be the first safety element to operate in case of abnormality.
Having the above-mentioned structure, conventional lithium secondary batteries involve the following problem. When a short-circuit occurs in the inside of the conventional lithium secondary battery to raise the temperature, the battery is incapable of suppressing an increase in short-circuit current.
In case where a short-circuit occurs in the inside of the lithium secondary battery to raise the temperature, a separator made of polyethylene or polypropylene interposed between a positive electrode and a negative electrode is expected to soften or melt to clog the pores of the separator, whereby the separator would exude the nonaqueous electrolytic solution contained therein or seal the nonaqueous electrolytic solution within itself to reduce its ion conductivity thereby to diminish the short-circuit current. However, the part of the separator distant from the heat generating part does not always melt. Besides, in case temperature rises, it is likely that the separator melts and flows to lose its function of electric insulation between positive and negative electrodes, which can lead to a short-circuit.
In particular, in the case of a lithium ion secondary battery, the negative electrode is prepared by coating a substrate functioning as a current collector, such as copper foil, with a slurry comprising a negative electrode active material such as graphite, a binder such as polyvinylidene fluoride (PVDF), and a solvent, and drying the coating layer to form a film. The positive electrode is similarly prepared in a film format on a substrate functioning as a current collector, such as aluminum foil.
The positive electrode comprises a positive electrode active material, such as LiCoO2, a binder, and a conducting agent. The conducting agent is to enhance electron conductivity of the positive electrode in case where the active material has poor electron conductivity. The conducting agent to be used includes carbon black (e.g., acetylene black) and graphite (e.g., artificial graphite KS-6, produced by Lonza).
When the temperature of such a battery increases to or above the temperature at which the separator melts and flows due to, e.g., an internal short-circuit, a large short-circuit current flows between the positive and negative electrodes at the part where the separator flows. It follows that the battery temperature further increases by heat generation, which can result in a further increase of the short-circuit current.
The invention has been made in order to solve the above-described problem. An object of the invention is to provide an electrode which increases its resistivity with temperature, a method of fabricating the electrode, and a battery using the electrode.
A first electrode according to the invention is an electrode having an electron conductive material layer of an electron conductive material comprising a conductive filler and a resin and an active material layer formed on the electron conductive material layer, the electron conductive material increasing its resistivity with a rise in temperature, which is characterized in that the proportion of the conductive filler in the electron conductive material is from 55 to 70 parts by weight.
According to this aspect, since the proportion of the conductive filler in the electron conductive material is 55 to 70 parts by weight, the rate of change in resistivity of the electrode can be increased. A battery constituted by using the electrode has an increased discharge capacity and is capable of reducing a short-circuit current.
A second electrode according to the invention is characterized in that the resin has a melting point ranging from 90xc2x0 C. to 160xc2x0 C. Since a resin having a melting point ranging from 90xc2x0 to 160xc2x0 C. is used, the resistivity increases at a certain temperature or thereabouts within the range of from 90xc2x0 to 160xc2x0 C.
A third electrode according to the invention is characterized in that the electron conductive material has an average particle size of from 0.05 xcexcm to 100 xcexcm. The particle size of the electron conductive material ranging from 0.05 to 100 xcexcm, the electrode increases its resistivity at around a prescribed temperature, and a battery using the electrode has an increased discharge capacity.
A fourth electrode according to the invention is characterized in that the conductive filler is a carbon material or a conductive non-oxide. Containing a carbon material or a conductive non-oxide as a conductive filler, the electrode has enhanced conductivity.
An fifth electrode according to the invention is characterized in that the resin is a crystalline resin. Containing a crystalline resin, the electrode has a further increased rate of change in resistivity at a prescribed temperature or thereabouts.
A first battery according to the invention is a battery having a positive electrode, a negative electrode, and an electrolytic solution provided between the positive and the negative electrodes, which is characterized in that the positive or negative electrode is any one of the above-described first to fifth electrodes. According to this aspect, since any of the first to fifth electrodes is used as the positive or negative electrode, the electrode increases the resistivity in case where the inner temperature of the battery rises to or above a prescribed temperature, thereby to reduce a short-circuit current. Therefore, the battery has improved safety.
A first method of fabricating an electrode according to the invention is characterized by comprising the steps of:
(a) pulverizing an electron conductive material containing a conductive filler and a resin,
(b) dispersing the resulting ground electron conductive material to make a paste,
(c) drying the paste to form an electron conductive material layer,
(d) dispersing an active material to prepare an active material paste, and
(e) applying the active material paste on the electron conducive material layer and pressing at a prescribed temperature under a prescribed pressure.
According to the process comprising the steps (a) to (d), the adhesion between the electron conductive material layer and the active material layer is improved thereby to reduce the contact resistance between the electron conductive material layer and the active material layer. As a result, the electrode prepared has a reduced resistivity.
A second method of fabricating an electrode according to the invention is characterized by comprising the steps of:
(a) pulverizing an electron conductive material containing a conductive filler and a resin,
(b) dispersing the resulting ground electron conductive material to make a paste,
(c) drying the paste and pressing the dried paste at a first temperature under a first pressure to form an electron conductive material layer,
(d) dispersing an active material to prepare an active material paste,
(e) drying the active material paste, and
(f) laying the dried active material paste on the electron conducive material layer and pressing at a second temperature under a second pressure to form an active material layer on the electron conductive material layer.
According to the process comprising the steps (a) to (f), the adhesion between the electron conductive material layer and the active material layer is improved thereby to reduce the contact resistance between the electron conductive material layer and the active material layer. As a result, the electrode prepared has a reduced resistivity.
A third method of fabricating an electrode according to the invention is characterized in that the prescribed temperature is the melting point of the resin or thereabouts. Since the prescribed temperature is the melting point of the resin or thereabouts, the adhesion between the electron conductive material layer and the active material layer is further improved thereby to further reduce the contact resistance between the electron conductive material layer and the active material layer. Further, the connection among the electron conductive material particles in the electron conductive material layer is improved thereby to reduce the resistance of the electron conductive material layer. Thus, the electrode prepared has a further reduced resistivity.
A fourth method of fabricating an electrode according to the invention is characterized in that the first temperature or the second temperature is the melting point of the resin or thereabouts. Since the first or second temperature is the melting point of the resin or thereabouts, the adhesion between the electron conductive material layer and the active material layer is further improved thereby to further reduce the contact resistance between the electron conductive material layer and the active material layer. Further, the connection among the electron conductive material particles in the electron conductive material layer is improved thereby to reduce the resistance of the electron conductive material layer. Thus, the electrode prepared has a further reduced resistivity.